For decades, postage meters have imprinted their postal indicia on envelopes by means of relief printing using printing dies. The indicia are generally formed with fluorescent ink of a distinctive color. Postage meters to serve such purposes are well known and reliable.
It has been proposed in recent years by some postal authorities to discontinue the use of die-printing postal indicia and instead to use off-the-shelf ordinary computer printers such as inkjet printers and laser printers for the printing of postal indicia. The use of off-the-shelf printers presents, of course, the profound problem of counterfeit indicia printed by parties wishing to print postage without having to pay for it. In an effort to reduce this problem, postal authorities have proposed to include within the postal indicia cryptographic information which is intended to permit the postal authorities to distinguish between counterfeit indicia on the one hand and legitimate indicia on the other hand. It is often proposed that the cryptographic information be printed on mail pieces by means of two-dimensional bar codes. Each such bar code contains information, such as CRC checksum, which serves to indicate whether the bar code has been correctly read.
The cryptographic authentication provides meaningful protection against counterfeit indicia only if the postal service treats authentic indicia differently than it treats counterfeit indicia. If mail pieces bearing counterfeit indicia are delivered by the postal service just as legitimate mail pieces are delivered, then this would become widely known and fraud would increase to high levels (given that the required printers are readily available).
A related problem is that cryptographic postal indicia, if printed in the form of two-dimensional bar codes, are not always easy to read. They will get smudged and smeared. They will be printed at skew angles relative to the edges of the mail pieces. They will have less than optimal contrast ratios. This presents the problem of what the postal service should do if it is unable to read a particular indicium on a mail piece.
It is instructive to discuss what counts as a “readable” bar code. Bar codes used in this context will contain a cyclical redundancy check, a checksum, a hash total, or some other test of the internal consistency of the bar code. As is well known to those skilled in the art, when the bar code is being generated, the “body” or text or content of the bar code is passed through a predetermined function. The function is preferably a cyclical redundancy check (CRC) polynomial but could less preferably be a checksum or hash function. The output of the function (for example, a CRC checksum) is noted and is written in the bar code along with the content. A bar code reader will read the body and the CRC checksum, and will pass the body through the same function yielding an output. This output is compared with the CRC checksum that was read from the bar code. If the bar code tests out to be internally inconsistent (for example by failing the CRC check) then we define this to mean that the bar code is “unreadable”. If, on the other hand, the output matches the CRC checksum that was read from the bar code, then we define this to mean that the bar code is “readable”.
In the case of an unreadable bar code, should the postal service deliver the mail piece anyway? Such an approach would encourage fraud. Persons with fraudulent intent would quickly learn to create bar codes which intentionally failed the CRC check so that they would be delivered without the nuisance of passing a cryptographic authentication.
In the case of an unreadable bar code, should the postal service return the mail piece to the sender? Given that many events, such as smudging or smearing, can male a bar code unreadable, such an approach would motivate mailers to use other franking means such as postage stamps or (if they are not outlawed) relief-type postage meters using printing dies. This awkward decision would repeat itself over a billion times a day in the United States where the daily mail volume is well in excess of a billion mail pieces daily, with a non-negligible percentage of mail pieces halving been rendered unreadable clue to smudging or smearing.
It is all too easy simply to say that the postal service would use bar-code readers with extremely high resolution and sophisticated software to deal with skew, poor contrast, and smudged and smeared indicia. Such bar code readers are very expensive. But even if modest-quality bar code readers were used, estimates of the cost to provide bar-code readers and authentication equipment for the United States are in the billions of dollars. Equipping every US Postal Service mail processing facility with high-quality readers instead of moderate-quality readers would put the nationwide installation cost at tens or hundreds of billions of dollars.
It would be extremely desirable to have an approach for the authentication of mail pieces bearing bar-coded indicia which would be reliable, inexpensive, and robust.